Absorption Transport and Storage

Iodine is usually ingested as an iodide or iodate compound and is rapidly absorbed in the intestine. Iodine entering the circulation is actively trapped by the thyroid gland. This remarkable capacity to concentrate iodine is a reflection of the fact that the most critical physiological role for iodine is the normal functioning of the thyroid gland. Circulating iodide enters the capillaries within the thyroid and is rapidly transported into follicular cells and on into the lumen of the follicle. This active transport is likely to be based on cotransport of sodium and iodine, allowing

Leaching from:

• rain and flooding

• deforestation

• glaciation

Return to land in rainwater: 0.0018-0.0085 mg l-

Return to land in rainwater: 0.0018-0.0085 mg l-

Iodine in plants: 0.01-1 mg kg-1

Leaching from:

• rain and flooding

• deforestation

• glaciation

Iodine in plants: 0.01-1 mg kg-1

Iodine in drinking water: 0.0001-0.1 mg l-1

Evaporation from oceans v -, Iodine in seafood: ' 0.3-3 mg kg-1

Iodine in Earth's crust: 0.05 ppm

Figure 1 Cycle of iodine in nature.

iodine to move against its electrochemical gradient. Several anions, such as thiocyanate, perchlorate, and pertechnetate, inhibit this active transport. There is evidence that the active transport clearly demonstrated in the thyroid gland is also true for extra-thyroidal tissues, including the salivary glands, mammary glands, and gastric mucosa.

In addition to trapping iodine, follicular cells also synthesize the glycoprotein, thyroglobulin (Tg), from carbohydrates and amino acids (including tyr-osine) obtained from the circulation. Thyroglobulin moves into the lumen of the follicle where it becomes available for hormone production. Thyroid peroxidase (TPO), a membrane-bound hem-containing glycoprotein, catalyzes the oxidation of the iodide to its active form, I2, and the binding of this active form to the tyrosine in thyroglobulin to form mono- or diiodotyrosine (MIT or DIT). These in turn combine to form the thyroid hormones tri-iodothyronine (T3) and thyroxine (T4). Thyroglobulin is very concentrated in the follicles through a process of compaction, making the concentration of iodine in the thyroid gland very high. Only a very small proportion of the iodine remains as inorganic iodide, although even for this unbound iodide the concentration in the thyroid remains much greater than that in the circulation. This remarkable ability of the thyroid to concentrate and store iodine allows the gland to be very rapidly responsive to metabolic needs for thyroid hormones. Figure 2 shows the structures of the molecules tyrosine and thyroxine.

Formation of thyroid hormones is not restricted to humans. Marine algae have an 'iodine pump' that facilitates concentration; invertebrates and all vertebrates demonstrate similar mechanisms to concentrate iodine and form iodotyrosines of various types. Although the function of these hormones in invertebrates is not clear, in vertebrates these iodine-containing substances are important for a variety

Tyrosine

Thyroxine (T4)

Figure 2 Structures of tyrosine and thyroxine (T4).

Figure 2 Structures of tyrosine and thyroxine (T4).

of functions, such as metamorphosis in amphibians, spawning changes in fish, and general translation of genetic messages for protein synthesis.

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